Electromagnetic micromotors of standard construction such as that shown on FIG. 1, which equip the majority of electromechanical watch movements of the analog or digital type, include a rotor constituted by a permanent magnet which is partially or indeed totally embedded in the interior of an overmolding formed of an injected plastic material.
Such rotor is mounted to be free in rotation in the interior of a generally cylindrical cage, the outer periphery of which is precision fitted in a stator.
Thus, for the moving assembly of the rotor, the overmolding includes at its two longitudinal extremities, spindles or pivots which are coaxial with the geometric axis of the rotor and are obtained with material from the body of the overmolding.
These spindles, which are thus also formed from injected plastic, are accommodated in corresponding housings arranged respectively in the bottom of the cage and in its cover.
Such type of motor to the present time has given entire satisfaction, in particular, on the one hand, thanks to the precision of the positioning of the rotor relative to the stator and on the other hand thanks to its extremely compact conception.
Nevertheless, its construction appears to be unsuited and limited whenever there is a question of incorporating it in movements of small dimension such as those for ladies' watches.
Effectively, when efforts are made to further miniaturize a watch movement, one is obliged to miniaturize the battery equipping such.
The diminution of the volume of the battery brings about a substantial reduction of the available energy.
Thus, in order to be able to offer the user sufficient autonomy of operation, it is necessary to limit the consumption of the movement.
One of the paths enabling response to this requirement is to increase the efficiency of the motor and this by diminishing the losses due to friction.
Such first problem being stated, it has been considered that a possible mechanical solution was to diminish the diameter of the rotor spindles in order to obtain a reduction of the friction couple.
Unfortunately, following numerous trials, the injection of these spindles has appeared to be almost impossible below a certain diameter.
Effectively, in most cases, the spindles have shown defects in structure and alignment which have rendered impossible their function as rotation guiding elements.
Furthermore, it has been possible to determine that below a certain threshold value, it was difficult and thus expensive to obtain the housings, respectively in the bottom and the cover of the cage, in which are accommodated the spindles or pivots of the rotor by drilling or by die forming because of the very frequent breakages and damage to the drill bits and punches. In view of these determinations, it has been logically presumed that the classic micromotor as described hereinabove could not be used to equip a watch movement which was further miniaturized, its limits of efficiency and dimensions being attained.
Thus, the invention has as purpose to remove this obstacle by proposing an electromagnetic micromotor of a conception close to that classic micromotor, i.e. constituted from an overmolding rotor pivotally mounted in a cage, but offering a higher efficiency so as to be capable of being energized with a sufficient autonomy of operation by a battery of lesser capacity.